1. Field of Invention
The present invention relates generally to a photolithographic processing method, and more particularly to a photolithographic processing method for manufacturing phase-shifting masks (PSM).
2. Description of Related Art
In photolithography, optical stepper machines, using ultra-violet light as a source of illumination, transfer patterns onto a wafer layer. The pattern transfer capacity of an optical stepper depends on two important parameters, namely, the resolution and the depth of focus (DOF). The resolution of an optical stepper depends primarily on the wavelength of the light source used. To obtain a quality exposure, the resolution needs to be small while the depth of focus needs to be large. Although a lower resolution is achieved when the wavelength of the light source is small, the depth of focus is correspondingly reduced. Hence, there is a trade-off between the resolution and the depth of focus in the design of an optical stepper system.
To satisfy the requirements for the next generation of 64MB DRAMs, a light source having a shorter wavelength must be developed for photolithographic processes. An example of such a light source is a krypton fluoride laser that generates a deep ultra-violet ray. Also being developed are photolithographic processes aimed at solving the resolution and depth of focus problem shown above, without using new light sources.
A phase-shifting mask, in contrast to a conventional mask, comprises a shifting layer, through which light is positively and negatively phase-shifted to simultaneously cause interference. As a result, the image pattern projected, via the shifting layer, onto the wafer layer to be patterned by the optical stepper has an improved resolution.
FIGS. 1(a) through 1(c) show the conventional method for manufacturing a phase-shifting mask. Referring to FIG. 1(a), a photomask layer 10 acts as the main body of the mask and comprises a material having good light transparency, such as quartz. An opaque shield 12, a phase shifting layer 14 and a photoresist layer 16 are sequentially formed above photomask layer 10. Opaque shield 12 and phase-shifting layer 14 are thin film layers made from materials, such as chrome or chromium oxide. Further, by adjusting the densities of the materials making up opaque shield 12 and phase-shifting layer 14, both opaque shield 12 and phase-shifting layer 14 can have a different light-penetrating power.
Referring to FIG. 1(b), photoresist layer 16 is patterned using a conventional photolithographic process, and phase-shifting layer 14 is dry etched using, for example, a reactive ion etching (RIE) method, to form an opening 11 that exposes a portion of opaque shield 12.
As shown in FIG. 1(c), a wet etching method subsequently removes a portion of opaque shield 12 covered by phase-shifting layer 14 and the portion of opaque shield 12 exposed by opening 11, so to expose a portion of photomask layer 10. Finally, as shown in FIG. 1(d), the manufacturing process is complete upon removal of photoresist layer 16. Hence, an uncovered light passing region 13 and interference regions 15 covered by phase-shifting layer 14 are formed inside opening 11.
The feasibility of applying the conventional method of manufacturing phase-shifting masks using photolithographic techniques primarily depends on the quality of phase-shifting layer 14. Therefore, it is critical to precisely control the output dimensions of the interference regions 15. However, the conventional manufacturing method uses wet etching process to form interference regions 15 of the phase-shifting mask, making it difficult to control the size and shape of the interference regions.